Electric motors and motor drive systems to drive the electric motors are used in a multitude of applications, including applications in which the motors drive variable torque loads. Example variable torque loads include fans and compressors in HVAC systems. Motor drive systems are also used in fluid pumping systems. Generally, in motor drive systems a sensor measures a characteristic of the load, and the motor drive controls the motor to maintain the characteristic near its setpoint. In some systems, the motor drive measures pressure in a fluid circuit (e.g. water pressure, air-flow pressure, vacuum) and controls the speed of the motor to maintain the pressure near its setpoint.
In a water pumping system, the pump must operate against the well head. Wells can be deep or shallow, and the fluid level within each well varies. The pump pressure can therefore be significantly higher than the pressure sensed by the pressure sensor. The pump pressure can vary significantly between installations and also within the same installation as the level of the fluid in the well rises or falls.
In fire protection, cooling towers, and other systems the pump must operate even if the motor drive fails.
Improvements are desirable to overcome the limitations of existing motor drives and systems in which motor drives are utilized.